The present invention pertains generally to systems for detecting and tracking target objects. More particularly, the present invention pertains to systems capable of detecting target objects through foliage. The present invention is particularly, but not exclusively, useful as a low frequency ( less than 500 MHz) detection system having a coherent sparse aperture that is capable of high resolution at low bandwidths.
In battlefield situations it is highly desirable to be able to locate target objects through foliage. More specifically, high quality battlefield awareness requires high resolution at low minimum detectable velocities (MDV). To penetrate foliage with an electromagnetic signal, low frequency systems are required. At these low frequencies, however, single platform sensors generally require a large array having apertures of several meters to achieve high azimuth resolution target detection. As one might expect, these bulky single platform sensors are burdensome in most battlefield situations.
In addition to requiring a large array, single platform sensors also require a large signal bandwidth, on the order of several megahertz, to obtain an adequate range resolution. Obviously, the amount of bandwidth that is available at the low frequencies required for foliage penetration is limited due to the crowded spectrum. For battlefield applications, the limited amount of low frequency bandwidth must often be shared by detection systems and non-detection systems. Thus, for battlefield applications, sufficient bandwidth is generally unavailable at foliage penetrating frequencies to obtain an adequate range resolution using a single platform sensor. In short, single platform systems having foliage penetration capability are bulky and require a large bandwidth.
In light of the above, it is an object of the present invention to provide a battlefield detection system capable of detecting and tracking target objects through foliage. Another object of the present invention is to provide a low frequency detection system that is capable of high resolution using relatively low bandwidth signals. Still another object of the present invention is to utilize distributed vehicles commonly found on a battlefield (i.e. tanks, light armored vehicles, manned and unmanned air vehicles, aerostats, etc.) to establish a coherent sparse aperture for a detection system. Another object of the present invention is to provide a high resolution battlefield detection system that uses only small, lightweight sensors. Yet another object of the present invention is to provide a battlefield detection system that provides accurate measurements of the relative elevation of the target. Another object of the present invention is to provide a battlefield detection system that resolves the location of a target object with little or no sidelobe ambiguities. Yet another object of the present invention is to provide systems for detecting and tracking target objects on a battlefield that are relatively easy to employ, simple to use, and comparatively cost effective.
A system and method for detecting and locating a target object through foliage includes a transmitter and a plurality of sparsely arrayed sensors. For the present invention, the transmitter is preferably configured to generate a foliage penetrating electromagnetic signal that has certain characteristics. Specifically, the transmitted signal preferably has a frequency that is less than approximately eight hundred megahertz (800 MHz) and a signal bandwidth of less than approximately one hundred kilohertz (kHz). In a preferred embodiment of the present invention, the transmitted signal is pulsed.
In operation, the transmitted signal is directed toward the suspected location of a potential target object for reflection from the potential target object. For the present invention, the sparsely arrayed sensors are pre-positioned at known locations to receive the reflected signal from the target. Importantly, the sensors are dispersed from each other to increase the angle, xcex1, between sensors (i.e. the angle established with the target object location defining the angle vertex). Stated another way, it is a feature of the present invention that the resolution of the detector system increases, as the angle, xcex1, between the sensors increases from zero degrees. With the large angle, xcex1, between the sensors, the system is capable of relatively high resolution with low minimum detectable velocity (MDV) while using the relatively low bandwidth transmitted signal.
It is contemplated for the present invention that each sensor will be small, lightweight and easily attachable to an existing battlefield vehicle such as a tank, light armor vehicle, unmanned air vehicle (UAV), etc. As such, the sensors will become mutually dispersed as the respective vehicles disperse creating the large angle, xcex1, described above. When an accurate measurement of the target object""s relative elevation is required, at least one of the sensors is preferably elevated from the remaining receivers to establish a vertical aperture for the system. For the present invention, each sensor receives and processes a common, coherent signal reflected from the target. Upon receipt of the reflected signal, each sensor extracts signal information from the signal for relay to a central processing site. The signal information can include frequency-related, phase-related and amplitude-related measurements.
The system further includes a processor that is positioned at the central processing site. In a preferred embodiment of the present invention, a wireless link is established between the central processing site and each sensor. This wireless link is then used to communicate signal information from each sensor and the location of each sensor to the central processing site. At the central processing site, the processor receives the sensor locations and the signal information from each sensor and coherently processes the signal information to obtain a set of possible locations for the target object. In addition, the processor performs a beamforming algorithm (i.e. an algorithm to reduce sidelobe ambiguities) to isolate the true location of the target from the set of possible locations. In a preferred embodiment of the present invention, a Maximum Likelihood Method (MLM) beamforming algorithm, as disclosed in detail below, is used to reduce sidelobe ambiguities and isolate the true location of the target. In one implementation, the transmitter can be placed on a moving platform such as an aircraft to allow the formation of images using synthetic aperture radar processing (SAR).